This is a data pipeline problem, not a spreadsheet problem. When a user pulls 365 consecutive days of daily history for a broad set of ETFs and finds that XLK and XLRE stop at March 2nd while all other tickers continue to March 12th, the natural instinct is to blame the formula. But the formula didn't change between tickers. The data source did.
The most likely explanation is that the provider feeding those two tickers stopped reporting on March 2nd for reasons related to the underlying index. XLK tracks the Technology Select Sector Index. XLRE tracks the Real Estate Select Sector Index. Both are maintained by S&P Dow Jones Indices, and both underwent a significant rebalance in March of this year. When an index rebalances, the constituent list changes. If the data vendor's agreement or pipeline for those specific indices was interrupted during that transition, the historical feed could have been cut off at the date of the rebalance. That would leave a clean 363-day record up to March 2nd and nothing after.
For the user building this comparison sheet, the practical implication is straightforward: you cannot trust that a 365-day window is consistent across tickers unless you verify the end date for each one individually. The gap is not random. It is specific to sector-tracking ETFs that depend on index-level data agreements. If you are building a model that requires identical time windows for every series, you need to trim the entire dataset to the shortest common end date, in this case, March 2nd. Or you need to switch to a data source that guarantees uniform coverage for sector indices, such as direct feeds from the exchange or a paid financial data API.
This is not a failure of spreadsheets. It is a failure of assumptions about data continuity. The user did the right thing by noticing the inconsistency and asking why. The answer is that data pipelines have seams, and those seams show up at rebalance dates. The lesson for anyone building similar comparison sheets: always audit the last date in every series before you run a single formula. A spreadsheet is only as reliable as the last row of data it received.